M66221FP データシートの表示（PDF） - MITSUBISHI ELECTRIC

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M66221FP

256 x 9-BIT MAIL-BOX

MITSUBISHI ELECTRIC  

MITSUBISHI 〈DIGITAL ASSP〉

M66221SP/FP

256 × 9-BIT MAIL-BOX

FUNCTIONAL DESCRIPTION

Arbitration Function

The M66221 has asynchronous accessibility from two independent

ports to shared memory, thus remarkably improving the throughput

of the entire processor system used in the multiport mode. On the

other hand, this accessibility causes a problem of contending for

selecting the same address in shared memory during the addressing

from both ports.

If the same address is contentionally selected, the following four basic

operations are possible depending on an access mode set from both

ports:

(1) A port .......... Read

B port .......... Read

(2) A port .......... Read

B port .......... Write

(3) A port .......... Write

B port .......... Read

(4) A port .......... Write

B port .......... Write

In this case, when both ports are operating in the read mode as given

in (1), correct data is read to both ports and the contents of memory

are not destroyed. There is no special problem. If the other port is in

the read mode while one port is operating in the write mode as given

in (2) or (3), however, data is written correctly but the data read from

the other port in the read mode may change during the same cycle.

This comes into question. When both ports are operating in the write

mode as given in (4), reverse data is written into each port and the

contents of memory may become uncertain. Consequently, no result

will be guaranteed.

The M66221 incorporated an arbitration function circuit to solve such

problems when contentionally selecting an address from both ports.

The arbitration function decides which of A and B ports determines

an address first, and unconditionally assigns access priority to the

first-in port (At this time, the Not Ready signal holds “H”). As for the

last-in port operation, the function inhibits any write to that port from

MPU at the same time when “L” is output to the Not Ready output pin

at the port regardless of a read or write operation during the period of

address matching of both ports. If the address of the first-in port

changes after that and both ports do not have the same address, the

Not Ready output is reset to “H” and the access in the stopped state

is accepted from the last-in port. If the same address is selected by

an address input from both ports simultaneously, a decision by the

arbitration function on the chip also affords access only from one

port, and outputs “L” to the Not Ready output for the other port

invalidate any access from MPU. Tables 3 and 4 give the relationship

between the port arbitration function and port access.

3

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